Protein kinase, also known as protein phosphakinase, is a kind of enzyme for catalyzing protein phosphorylation reaction. Such enzyme can transfer γ-phosphoric acid on ATP to the amino acid residue of protein molecule so as to change the conformations and activities of protein and enzyme. Phosphorylation of protein is an important step in transduction of multiple signals, and most of important life activities in the cell depend upon the phosphorylation of protein. The enzyme is a key factor for regulating cell signals including cell proliferation and cell differentiation.
The protein kinase signal has two important roles in transduction: one is to regulate the activity of protein by phosphorylation, phosphorylation/dephosphorylation is the common mechanism for reversible activation of most signal path components, some proteins are active after phosphorylation and some are active after dephosphorylation; the other is to gradually amplify signal and cause cell reaction by gradual phosphorylation of proteins.
Janus-activated kinase\signal transducer and activator of transcription (JAK-STAT) is a recently discovered signal transduction path in cells closely related to cytokines, involving in many important biological processes such as cell proliferation, differentiation, apoptosis and immunoregulation. Janus-activated kinase is a non-receptor tyrosine protein kinase which has 4 family members including JAK1, JAK2, TYK2 and JAK3/. JAK is a kind of very important drug targets. It has been proven that JAK inhibitor can be used for preparing medications for treating blood system diseases, tumor, rheumatoid arthritis, psoriasis, etc. at present. Since JAK inhibitor has obvious medical use and can be used for preparing medications for treating various diseases, it is extremely useful for studying and finding such compound.
China patent application No. CN 102026999A discloses an azetidine and cyclobutane derivative and the combination, use and preparation method thereof. They can be JAK inhibitors used for treating JAK-related diseases such as inflammatory diseases, autoimmune diseases and cancer. The parent nucleus structure is shown as the following chemical formula. As an effective JAK inhibitor, it can be taken as a bulk drug for preparing medications for treating diseases such as rheumatoid arthritis, dermatosis, cancer and myeloproliferative disease.

Currently, Tofacitinib of Pfizer has been listed as a typical JAK inhibitor and it is the pioneered drug for treating rheumatoid arthritis. Nevertheless, the treatment field of JAK inhibitor involves different aspects, and the direction of research is to find more new compounds with strong activity and high druggability.